rechargeable li-ion batteries based on olivine-structured (lifepo4) cathode materials

1

Arun Kumar Department of Physics and Institute for Functional Nanomaterials, University

of Puerto Rico, San Juan, PR-00931

Rechargeable Li-ion batteries based on Olivine-structured (LiFePO4) cathode materials

20 August 2012 UPRRP

2

IntroductionSome basics!Emergence of Li-(ion) rechargeable batteriesCurrent status of cathode materials

Motivation Experimental details Results and discussions Summary

Outline

3

About Batteries

Introduction

Chemical reactions leads to electrical energy Convert chemical energy to electrical energy

Collection of ´cells´ to get the required voltage

A Cell is comprised of Cathode (Undergoes reduction)

Anode (Undergoes oxidation)

Electrolyte ( ionic conductor)

Active component

Inactive component

4

Cells

Primary Chemical reaction is irreversible

Reactions eat up the active material

Schematic of a primary half cell

Anode Cathode

A A+ + e C + e C-

EA,Oxidation EC,Reduction

ELECTROLYTE

ECELL = EA,OX+EC,RED

Some basics

Topic of this talk

SecondaryChemical reaction is reversible

Active material remains!!

6

Rechargeable lithium ion battery : Schematic

Li ion shuttles between anode and cathodeDuring charging Li ions move from cathode to anode During discharge Li ions gets intercalated into cathode

X Li + + x e - + Li yM X Li y+x MXCharge

DischargeX Li + + x e - + Li yM X Li y+x MX

Charge

Discharge

G= -nFEFor maximum (+ve) E, cathode has to be highly oxidizing and anode has to be highly reducing

Cathode –Intercalation compound

Anode - Li metal (Li battery) not safe

or another intercalation compound (Li ion battery)

7

Theoretical capacityImportant concept in reference to the quality of the active material

Faraday’s first law of electrolysis:F = N.e

Gram equivalent = MW/nF = 965000 C 26.8 Ah

LiCoO2 : MW = 99.9 and n =1 so, 99.9 g of LiCoO2 is capable of delivering 26.8 Ah

i.e. theoretical capacity of LiCoO2 is 26.8Ah/99.9 g = 268 mAh/g

LiCoO2+ Li+ + e- LiCoO2

LiC6 Li++ C6+ e-

Theoretical 1 e-

Practical 0.5-0.6 e-

Out put voltage : 3.6 V NO of e- or Li+

Molecular weight (Kg)

How to optimize such system ?

8

Electrolyte

eVoc = μA -μC <Eg

At high current densities, the ionic motion within an electrode and/or across an electrode/electrolyte interface is too slow for the charge distribution to reach equilibrium, that is why the reversible capacity decreases with increasing current density in the batteryThe electrolyte must satisfy several additional requirementssuch as:A Li-ion conductivity σLi>10-4 S/cm over the temperature range of battery operation. An electronic conductivity σe<10-10 S/cm.Chemical stability with respect to the electrodes, including the ability to form rapidly a solid/electrolyte-interface (SEI) layer where kinetic stability is required because the electrode potential lies outside the electrolyte window. Safe materials, i.e., preferably nonflammable and non explosive if short-circuited. Low toxicity and low cost.

9

Energy density (Wh) = Capacity (Ah) Voltage (V)Large work function (highly oxidizing)

+ maximize cell voltage. Insertion/extraction of a large amount of lithium

+ maximize the capacity.High cell capacity + high cell voltage = high energy density Reversible lithium insertion/extraction process.

+ make it rechargeable No structural changes

+This prolongs the lifetime of the electrode. Good electronic and Li+ ionic conductivities.

+ improves the rate capability Chemically stable over the entire voltage range

+ No reaction with the electrolyte. Inexpensive, environmentally benign and light weight.

+safe, friendly, portability

Cathode: Material requirements

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Price, safety and low toxicity are strong arguments that Fe based cathode should provide significant technological advantages in a Li-ion system over LiCoO2 or LiNiO2 systems But..All are not good with Fe based cathode material??? Low electronic conductivity!!

Candidates for cathodes

CompoundCapacity:

theo./Pract. (mAh/gm)

Working Voltage(V)

CycleabilityEnergy density

( Wh/kg)Conductivity Drawbacks

LiMn2O4 148/120 3.8 300 100 10-5S/cm Severe capacity fading

LiMnO2 285/140 4.0Very difficult to stabilize in layered structure

LiCoO2 270/140-150 3.7 400 180 10-3S/cm Expensive, toxic

LiNiO2 270/140 3.9Severe cation mixing, thermal instability at the charged state

LiFePO4 170/165 3.4 1000 130 10-10 S/cm Low electronic conductivity

11

Structure of olivine LiFePO4

The structure consists of corner-shared FeO6 octahedral and edge-shared LiO6 octahedra running parallel to the b-axis, which are linked together by the PO4 tetrahedral .

The triphylite LiFePO4 belongs to the olivine family of lithium ortho-phosphates with an orthorhombic lattice structure in the space group Pnma.

Structure(using ICSD software)

14

LiCoO2 and their substituted versions adapt the α-NaFeO2-type structure (space groupR3 m ), which is a layered, rhombohedral structure in which the lithium ions can move quite freely in the two-dimensional planes perpendicular to the c-axis. In this structure, ~0.6 Li can be extracted and inserted during the charge and discharge cycles, Further extraction leads to irreversible collapse of the structural framework

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Outline

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• High Performance: Provides a High Theoretical capacity of ~170 mAh/g and a High Practical capacity as high as ~ 165 mAh/g. • Extremely Safe/Stable Chemistry

High intrinsic safety, non-explosive High thermal stability• High Discharge Rate Capability • Extraordinary Long Cycle Life• No memory effect• Environment Friendly• Non-toxic, non-contaminating-no rare metals• Wide working temperature range • From -45°C to 70°C ( Extremely cold and extremely hot

weather will not affect its performance)

Motivation Realization of high electronic conductivity in LiFePO4

17

ImprovingConductivity of

LiFePO4

SONY (2001)

• Synthesis of small particle LiFePO4 (50-100nm)

Armand et. al. (2001)

• Carbon coating of LiFePO4 particle (carbon~ 4-12%)

L.Nazar el. al. (2001)

•C-matrix composite influence on LiFePO4

Y-M.Chiang et. al. (2002)

•Doped LiFePO4 (Nb5+Zr4+Al3+)

8 – fold order of magnitude increase in conductivity )

Advances in the conductivity of LiFePO4

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Nanomaterial structureAdvantage The smaller particle size increases the rate of lithium insertion/extraction because of the short diffusion length for lithium-ion transport within the particles, resulting in enhanced rate capability.The smaller particle size enhances the electron transport in the electrode, resulting in enhanced rate capability.The high surface area leads to enhanced utilization of the active materials, resulting in higher capacity.The smaller particle size aids a better accommodation of the strain during lithium insertion/extraction, resulting in improved cycle life Disadvantage Complexities involved in the synthesis methods employed could increase the processing cost, resulting in higher manufacturing costHigh surface area may lead to enhanced side reactions with the electrolyte, resulting in high irreversible capacity loss and capacity fade during cyclingThe smaller particle size and high surface to volume ratio could lead to low packing density, resulting in low volumetric energy density

Electrochemical impedance spectroscopy (EIS) can reflect the electrochemical characteristics and inner structure more accurately .

19



Outline

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Synthesis of LiFePO4

Ammonium dihydrogen phosphate

Lithium carbonate

High Energy Ball Milling (8-24H) ,(500 rmp 10h)

Organic removal at 350°C- 8-12h

Sintering at 650°C-3 h (crystallization)

Iron Oxalate dihydrate

Carbon coated Powder synthesis through solid state route

Carbon coated ( HEBM) RTA effect

Zinc oxide

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Schematic of CR2032 coin cell

Working electrode (cathode) Active powder – 80 wt% Carbon black – 10 wt% PVDF binder – 10 wt% Current collector- Al

Electrolyte LiPF6 – 1M EC : DMC- 1:1Anode Li foil

Structural Characterizations: X-ray diffraction, Raman spectroscopyElectrochemical Characterizations: Cyclic voltammetry, Charge discharge , rate -capability and EIS study

Coin cell assembled in our group

Experiment: coin cell fabrication

22



Outline

23

X-ray diffraction analysis-I

15 20 25 30 35 40 45

C/LiFePO4

221,

041

022

012

14021

113

1

031

20011

110

1120

011

Inte

nsity

(au)

2 (degree)

020

(a)

LiFePO4

Li3Fe2(PO4)3 impurities indicating incomplete reduction of Fe3+ to Fe2+.Refined parameters

Phase pure orthorhombic LiFePO4 + complete reduction of Fe3+ to Fe2+.

15 20 25 30 35 40 45

221,

041

022

01214

0211

131

031

20011

110

1120

011

Inte

nsity

(au)

2 (degree)

020

15 20 25 30 35 40 45 50 55

Inte

nsity

(a.u

.)

2

(a) 8h (b)12h (c) 16h (d) 20h (e) 24h

(a)

(b)

(c)

(d)

(e)

24

20 30 40 50 600

100

200

300

400

500

600

700

800

42.0 42.5 43.0

0

20

40

60

80

100

120

140

160

Inte

nsity

(a. u

.)

2

*

Inte

nsity

(a.u

.)

2

**

* Fe2P

Carbon coating and rapid thermal annealing effects

Fe2P is an impurity, which is highly conducting . It helps to improve the Electrochemical properties.

15 20 25 30 35 40 45 50 55

LiFe0.094Zn0.06PO4/C

2

Inte

nsity

(b)

LiFe0.97

Zn0.03

PO4/C

(a)

All the XRD peaks could be indexed based on a orthorhombic unit cell (Pnma).

No evidence of additional crystalline phases (crystalline carbon or other phases) were seen from the XRD.

The increase in the unit cell parameters is due to the formation of

LiFePO4.

X-ray diffraction analysis-II

25

X-ray diffraction analysis-III

10 15 20 25 30 35 40 45 50 55

0

200

400

600

800

Inte

nsity

(arb

.uni

ts)

2 (Degrees)

Iobserved ICalculated Iobs-Ical

(a)

Element Wyckoff x y z s.o.f Li 4a 0 0 0 1.00 Fe 4c 0.281 0.25 0.025 1.00 P 4c 0.095 0.25 0.582 1.00 O1 4c 0.097 0.25 0.258 1.00 O2 4c 0.457 0.25 0.781 1.00 O3 8d 0.342 0.537 0.215 1.00 Rwp (%) 17.9 Rp (%) 12.6 Goodness of Fit (GOF)

1.6

Lattice Parameters

a 10.314 Ǻ b 6.002 Ǻ c 4.693 Ǻ

The only phase observed is LiFePO4.

The Rietveld refinementindicates that the iron is completely ordered.

26

Morphological evolution : scanning electron microscopy

20kV 15,000 1μm

8H (a)

20kV 15,000 1μm

12H (b) 16H (c)

20kV 15,000 1μm

20kV 15,000 1μm

116H (c)

20kV 15,000 1μm

24H (e) • SEM images of the non carbonaceous LiFePO4.

• Particle attain a more compact structure without any substantial increase in size .

The particle size reduces slightly with higher ball milling time. There is no appreciable effect after 12h of ball milling.

8h 12h 16h

20H24h

20h

27

C• SEM micrographs of (a) LiFePO4/C RTA method,

(b) LiFe0.97Zn0.3PO4/C and (c) LiFe0.93Zn0.7PO4/C

• This suggests that the carbon binds strongly to the surface of precursors, thus coating the particle with carbon restricts further growth of the particles .

Morphological evolution : scanning electron microscopy

(a)(b)

(b)

(c)

The grain size reduced with Zn substitution up to 0.3%; more Zn resulted in agglomeration

28J.Of P. Source 178(2008)

•. Transmission electron micrographs of C- LiFePO4 composite particles calcined at 650°C showing morphological features with various magnifications

•TEM images shows the synthesized LiFePO4 powder are mainly fine particle less than <80nm.

•The image shows carbon – like nanometer sized webs wrapped around and connecting the LiFePO4 particle .

TEM analysis of C/LiFePO4

Schematic diagram illustrating how carbon is distributed and coated on the LiFePO4 particles

29

Raman Spectroscopy

a) Raman spectra of C-LiFePO4 powder at various stages of charge – discharge process in frequency range 150 to 1200 cm-1 (b) Raman spectra of C-LiFePO4 powder in the frequency range 600 to 1800 cm-1.

The Raman modes in the range of 900 to 1150 cm-1 (see figure b) are due to the stretching mode of PO4

3- unit and involve symmetric and asymmetric of P – O bonds . These Raman modes also show a systemic change in Raman intensity with electrochemical

cycling process i.e. Raman intensity of all the PO43- generated optical modes which decreased

during charging process and vive versa. The two prominent modes at ~ 1345 and 1587 cm-1 are the fingerprints of amorphous carbon.

The mode at ~ 1587 cm-1 is assigned to sp2 graphite like (G band) and the mode at ~ 1345 cm-1 is assigned to sp3 type amorphous carbonaceous material (D band).

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For pure LiFePO4 the anodic/cathodic peaks at scan rate are 3.5/2.9. The separation of redox peaks ΔV is 0.7 indicating that the electrochemical behavior is controlled by diffusion step .

The difference of redox potential peaks was 0.32 V for carbon coated LiFePO4 sample . The Li ion diffusion coefficient was ~ 7.13 x 10-14 cm2s-1 whereas in the case of pure

LiFePO4 it was merely ~ 1.28 ×10-15 cm2s-1 and 4.67 x 10-14 for LiFe0.97Zn0.3PO4 hence, the Li ion (de)intercalation was better in C-LiFePO4 and LiFe0.97Zn0.3PO4.

Electrochemical property: Cyclic voltagrams I

ip = 2.69 X 105 n3/2 C0b A DLi

1/2 υ1/2 Randles–Sevcik equation

Bull. Korean Chem. Soc. 2011, Vol. 32, No. 3

31

Electrochemical property: Cyclic voltagrams II

2.5 3.0 3.5 4.0 4.5-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Cur

rent

(mA

)

Voltage (V) vs. Li/Li+

Sample S2

Sample S1

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4-0.0006

-0.0004

-0.0002

0.0000

0.0002

0.0004

0.0006

LF0.93Zn0.07P/C

LF0.97Z0.03P/C

Cur

rent

(A)

Potential (V)

0.1mV/s

Slow cooling ΔV 0.32> LiFe0.93Zn0.7PO4/C ΔV 0.30>Fast cooling 0.29> LiFe0.97Zn0.3PO4/C ΔV 0.25

The difference of redox potential peaks was 0.32 V for sample S1 (slow cooling rate) and 0.29 V for sample S2 (fast cooling rates).

The separation of redox peaks is ΔV is 0.25 , 0.30 for LiFePO4/C , LiFe0.97Zn0.3PO4/C and LiFe0.93Zn0.7PO4/C indicates that the electrochemical behavior is controlled by diffusion step.

32

The flats voltage profile of about 3.4 V versus Li/Li+ is characteristic of olivine structure

Electrochemical property: Charge – Discharge I

0 10 20 30 40 50 60 70 80 902.22.42.62.83.03.23.43.63.84.04.24.44.6

Volta

ge (V

)

Capacity mAh/g

16h 12h 20h 24h

Id=I

c= 15mA/g

Electrochemical charge/discharge tests of the cathodes showed a significant improvement on forming nano sized material

0 20 40 60 80

2.4

2.8

3.2

3.6

4.0

4.4

Volta

ge (V

vs

Li/L

i+)

capacity mAh/g

1st ch

1st dis

5th ch

5th dis

15th ch

15th dis

2.5-4.5V

IC=Id= 15mA/g

0 10 20 30 40 50 60 70

2.4

2.8

3.2

3.6

4.0

4.4

Volta

ge (L

i/Li+

) (V)

capacity (mAh/g)

8HBM

Volatge 2.5-4.4

C/10

15mA/gCurrentt density

33

Electrochemical property: Charge – Discharge II

-20 0 20 40 60 80 100 120 140 1602.42.62.83.03.23.43.63.84.04.24.4

LF0.93Zn0.07P/C

Pote

ntia

l (V)

Capacity mAh/g

Cycled between2.5- 4.3Ic=Id=30 mA/g

1st discharge

20th discharge

High current density with moderate capacity was obtained with LiFe0.97Z0.3PO4/C compared LiFePO4/C

LiFe0.93Zn0.7PO4/C had inferior specific capacity and rate-capability compared to LiFe0.97Z0.3PO4/C and LiFePO4/C

LiFe0.93Zn0.7PO4/CIC=Id=30mA/g

-20 0 20 40 60 80 100 120 140 160

2.42.62.83.03.23.43.63.84.04.24.4

L iF eP O 4/C

I C = I d= 3 0 m A /g

0 20 40 60 80 100 120 140 1602.4

2.8

3.2

3.6

4.0

4.4

Pote

ntia

l (V)

Capacity, mAh/g

cycled between with I

C=I

D=30mA/gm

1st ch

20th ch

1st dis

20th dis

LiFe0.97Zn0.03PO4/CLiFe0.97Zn0.3PO4/C

340 5 10 15 20 25 30

0

10

20

30

40

Cap

acity

mA

h/g

no . of cylcle

15mA/g 30mA/g 45mA/g 120mA/g 15mA/g

0.7C 2.5C 6.6C 12C

0 5 10 15 20 25 30 35

20

25

30

35

40

45

50

55

60

ca

paci

ty m

Ah/

g

cycle no.

15mA/g 30mA/g60mA/g

Nanoparticle LiFePO4 show high rate capability

8HBM

12HBM

24HBM

0 10 20 30 40 50 60

20

40

60

80

Dis

char

ge C

apac

ity (m

Ah/

g)

Cycle number

0.2C 1.3C4.3C

0.2C

Electrochemical property: Rate Capability and Cyclability-I

35

cycle performance for prepared for samples LiFePO4 , LiFePO4/C and

0 5 10 15 20 2510

20

30

40

50

60

Dis

char

ge c

apac

ity m

Ah/

g

Cycle number

LiFe0.03Zn0.07PO4/C

LiFePO4/C

LiFe0.93Zn0.7PO4/C

0 10 20 30 4020

40

60

80

100

120

140

160

Dis

char

ge C

apac

ity, m

Ah/

g

Cycle number

Id= 60mA/g

C/2.5

0 10 20 30 4020

40

60

80

100

120

140

160

Dis

char

ge C

apac

ity, m

Ah/

g

Cycle number

Id= 60mA/g

C/2.5

Electrochemical property: Rate Capability and Cyclability-II

LiFe0.93Zn0.7PO4/C

36

LiFe0.93Zn0.7PO4/C had inferior specific capacity and rate-capability compared to LiFe0.97Zn0.3PO4/C and LiFePO4/C

Electrochemical property: Rate Capability and Cyclability-III

37

Electrochemical impedance spectroscopy in Li-Ion Batteries

RS

Rct1 Rct2

CPE1 CPE2

The equivalent circuit of the Li-ion battery

This curve can be used to determine the cell capacity, effect of the discharge-charge rate, temperature and information on the state of health of the battery.

(-X

)(1

/wc)

R

Ohmic

ACTIVATION PROCESSES

f Increasing

DIFFUSION (WARBURG BEHAVIOR)

Volta

ge

Discharge

OCVIR drop

Activization Polarization

Ohmic polarization

High rate discharge

End of life (concentration polarization)

38

Electrochemical impedance spectroscopy(EIS)

Fitted RC model for various EIS spectra where R1 electrolyte resistance, R2 surface layer resistance ,R3- charge transfer resistance , CPE- double layer capacitance and w- Warburg impedance.

100 150 200 250 300 350 400 4503.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

Volta

ge (V

)

Charge transfer Rct

0 100 200 300 400 5000

100

200

300

400

OCV3.45 0CV3.460 OCV3.47 OCV3.59 OCV3.92 OCV4.03

-Z(Im

g)[o

hm]

Z(Re) [ohm]

Charging applied voltage

-10 0 10 20 30 40 50 60 70 80

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

Volta

ge

Charge

IR drop

Activation Polarization

diffusion

39

Electrochemical impedance spectroscopy(EIS)

0 50 100 150 200 250 300 350 4000

50

100

150

200

250

300

OCV3.63 OCV3.432 OCV3.387 OCV3.206 OCV2.78

-Z(Im

g)[oh

m]

Z(Rel)[ohm]

Dischrge-apply Voltage

120 140 160 180 200 220 240 260 280 300

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

Volta

ge (V

)

Charge transfer resistance

B

The charge transfer resistance increases with the relaxation time accompanied by a drop in the measured OCV as indicated in the legend. This linear increase in charge transfer resistance implies a change in the electrode-electrolyte interface at the charged cathode with increasing relaxation time.

40

Charge applied Voltage

R1 (ohm.cm2) R2 (ohm.cm2) CPE(ohm.cm-2) n W

3.45 405.9 2.27E-05 0.716 2.53E-07

3.464 355.1 2.36E-05 0.715 3.10E-07

3.92 272.7 1.95E-06 0.749 1.05E-07

4.03 117.6 9.56E-06 0.796 3.56E-06

Discharging applied voltage

R1 R2 CPE n W

4.133 146 1.17E-05 0.793 3.88E-05

4.034 150 1.20E-05 0.787 8.50E-06

4.034 210 1.24E-05 0.785 80.99-06

4.129 280 1.00E-05 0.798 4.49E-09

4.108 290 1.01E-05 0.795 3.70E-09

EIS data

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IntroductionIntroductionSome basics!Some basics!Emergence of Li-(ion) rechargeable batteriesEmergence of Li-(ion) rechargeable batteriesCurrent status of cathode materialsCurrent status of cathode materials

MotivationMotivation ExperimentExperimental detailsal details Results and discussionsResults and discussions Summary

Outline

42

Summary and conclusion Pure and composite LiFePO4 material with olivine phase was successfully synthesized

using a solid state method

+Good reproducibility, high product yield , and short heat treatment.

+No evidences of additional crystalline

Residual carbon coating on the LiFePO4 particles ensured composite LiFePO4/C in a reducing atmosphere and which enhanced the inter-particle electronic conductivity and electrochemical properties.

+result in reduction to Fe2+ which help to improve the electrical conductivity CV analysis revealed an approximately 3.8 fold increase in diffusion constant when

the bare LiFePO4 was coated with carbon. The results from CV & in situ EIS suggested that the carbon coating reduced the

electrical resistance (Rct) at the particle surface during the charge-discharge cycles which led to enhance Li ion diffusion and electrochemical performances.

CV analysis revealed an approximately 1.8 fold increase in diffusion constant was achieved by complementary Zn doping.

Electrochemical charge/discharge and rate capability tests of the composite cathodes with LiFe0.97 Z0.3 PO4/C with 3% ZnO showed high rate capability and capacity compared to 7% ZnO and 10% carbon coating.

The results indicate that the Zinc atoms prevent the collapse of the LiFePO4 lattice structure. However, future studies may be desired.

43

Thank you

Sincere thanks to Dr Reji ThomasDr. Naba KaranDr. Jose Savadara

rechargeable li-ion batteries based on olivine-structured (lifepo4) cathode materials

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